HVDC (high-voltage direct current) electrical power transmission uses direct current for the transmission of electrical power. This is an alternative to alternating current electrical power transmission which is more common. There are a number of potential benefits to using HVDC electrical power transmission.
In order to use HVDC electrical power transmission, it is typically necessary to convert alternating current (AC) to direct current (DC) and back again. Historically this has involved a six pulse bridge type topology based on thyristors which is sometimes known as a line-commutated converter (LCC).
Recent developments in the power electronics field have led to an increased use of voltages-source converters (VSC) for AC-DC and DC-AC conversion. VSCs typically include multiple converter arms, each of which connects one DC terminal to one AC terminal. For a typical three phase AC input/output there are six converter arms, with the two arms connecting a given AC terminal to the high and low DC terminals respectively forming a phase limb. Each arm includes an apparatus which is termed a valve and which typically includes a plurality of sub-modules which may be switched in a desired sequence.
In one form of known VSC, often referred to as a six pulse bridge, each valve includes a set of series connected switching elements, typically insulated gate bipolar transistors (IGBTs), each IGBT connected with an antiparallel diode. The IGBTs of the valve are switched together to connect or disconnect the relevant AC and DC terminals, with the valves of a given phase limb being switched in antiphase. By using a pulse width modulated (PWM) type switching scheme for each arm, conversion between AC and DC voltage can be achieved.
In high voltage applications where a large number of series connected IGBTs are required the approach does however require complex drive circuitry to ensure that the IGBTs switch at the same time as one another and requires large passive snubber components to ensure that the high voltage across the series connected IGBTs is shared correctly. In addition the IGBTs need to switch on and off several times over each cycle of the AC voltage frequency to control the harmonic currents. These factors can lead to relatively high losses in conversion, high levels of electromagnetic interference and a complex design.
In another known type of VSC, referred to a modular multilevel converter (MMC), each valve includes a series of cells connected in series, each cell including an energy storage element, such as a capacitor, and a switch arrangement that can be controlled so as to either connect the energy storage element in series between the terminals of the cell or bypass the energy storage element. The cells are often referred to as sub-modules with a plurality of cells forming a valve module. The sub-modules of a valve are controlled to connect or bypass their respective energy storage element at different times so as to vary over the time the voltage difference across the valve. By using a relatively large number of sub-modules and timing the switching appropriately the valve can synthesise a stepped waveform that approximates to a sine wave and which contain low level of harmonic distortion. As the various sub-modules are switched individually and the changes in voltage from switching an individual sub-module are relatively small a number of the problems associated with the six pulse bridge converter are avoided.
In the MMC design the high side terminal of each valve will, at least for part of the cycle, be connected to a voltage which is substantially equal to the high DC busbar voltage, +VDC, whilst the low side terminal is connected to a voltage which is substantially equal to the low DC busbar voltage, −VDC. In other words each valve must be designed to withstand a voltage of 2VDC. This requires a large number of sub-modules with capacitors having relatively high capacitance values. The MMC converter may therefore require a relatively large number of components adding the cost and size of the converter.
In some applications the size or footprint of a VSC may be a particular concern. For example HVDC is increasingly being considered for use with offshore wind farms. The electrical energy generated by the wind farms may be converted to HVDC by a suitable VSC station for transmission to shore. This requires a VSC to be located on an offshore platform. The costs associated with providing a suitable offshore platform can be considerable and thus the size or footprint of VSC station can be significant factor in such applications.
Recently another form of VSC, referred to as a controlled transition bridge (CTB), has been proposed. FIG. 1 illustrates a known controlled transition bridge converter 100. This converter has six arms, each arm including a switch SW1-SW6. Each of the switches SW1-SW6, which will be referred to herein as director switches, may include a plurality of series connected switch elements (not shown). As with the six-pulse bridge described above each director switch SW1-SW6, which can be switched to be conducting or non conducting, connects one DC terminal, i.e. the high or low side DC terminal or busbar (DC+, DC−), to an AC node 102a-c, i.e. a node which is between the two director switches of a given phase and which represents a tap/feed point for the AC current/voltage. The converter 100 also includes a chain-link circuit 101a-c connected to a node between the two director switches forming a particular AC phase limb, (SW1, SW4; SW3, SW6 or SW5, SW2). In other words a first end of the chain-link circuit 101a-c is coupled to the respective AC node 102a-c. The other (second) end of each chain-link circuit is connected to ground. Each chain-link circuit includes a plurality of cells 103 connected in series. Each cell has terminals 104a, 104b for upstream and downstream connection and includes a capacitor 105 connected with four IGBTs 106 in a full H-bridge arrangement, each IGBT being connected with an antiparallel diode. DC link capacitances 107 are provided for the DC buses. Such a converter is described in WO2011/050847.
In operation, the two arms of a given phase limb may be switched between a high state, where the high side director switch, i.e. SW1, SW3 or SW5, is on, i.e. conducting, and the respective low side director switch, i.e. SW4, SW6 or SW2, is off, i.e. non-conducting, and a low state where the opposite is true. However, unlike in the conventional two level converter, in this converter the transition between the high and low states is controlled by the relevant chain-link circuit 101a-c. For example to transition from the high state to the low state for AC phase A, the cells of the chain-link circuit 101a are controlled to connect the capacitors in series to provide a voltage substantially equal to +VDC at the top of the chain-link circuit 101a (i.e. at the connection to the phase limb) whilst director switch SW1 is turned off. This means that there is substantially no voltage drop across director switch SW1 when it is turned off. The cells of the chain-link circuit can then be controlled to bypass the capacitors of the modules in sequence to ramp the voltage at the top of the chain-link circuit, and hence the voltage at the relevant AC node 102a, down to zero. As the cells of the chain-link circuit are based on a full bridge arrangement the capacitors can be connected to present a negative voltage at the top of the chain-link circuit. The cells of the chain-link circuit can therefore be connected in sequence to step the voltage at the top of the chain-link circuit down to −VDC. Once the voltage at the AC node 102a is substantially down to −VDC the director switch SW4 can be opened to enable to enable the low state. There is therefore a transition state between the high state and the low state (and vice versa) when the chain-link circuit is used to control the voltage transition at the AC node. It will be noted that during the transition the current for that phase may flow to ground via the chain-link circuit. Thus the DC current is interrupted and hence DC link capacitances 107 are provided to avoid excess voltage ripple on the DC terminals.
In its basic form the CTB produces a trapezoidal waveform of voltage with the chain-link circuit providing soft switching of the director switches of the six pulse bridge arrangement and with a controlled transition so that snubber circuits to provide voltage sharing during switching are not necessary. Also, as the director switches SW1-SW6 are switched with a low voltage difference across the switch, the requirements for the switching control of the various switching elements making up a director switch is relaxed compared with the six pulse bridge.
The CTB converter requires only three chain-link circuits, one per phase, as oppose to the MMC converter which requires a plurality of sub-modules for each arm. It will be noted that the cells 103 of the chain-link circuits 101a-c are based on a four-switch full-bridge arrangement, whereas the MMC cells may be based on a two-switch half-bridge arrangement. However it will be appreciated that the maximum voltage difference developed across a chain-link circuit during normal operation of the converter 100 is equal in magnitude to VDC. As mentioned above for an MMC the maximum normal voltage difference across each valve would be equal to 2VDC. This means that a controlled transition bridge converter may have a significantly lower footprint than an MMC.
The CTB type converter thus offers various advantages that would make such a converter attractive, especially for applications where size of the converter station is important. However there are some practical issues regarding implementation of such a converter.
The general control scheme results in a trapezoidal waveform at the connection point between the two converter arms of a phase limb. To allow for AC voltage magnitude control in such a scheme tap changers may be integrated into the power transformer. However this results in a problem in terms of fault handling where the AC voltages become extremely unbalanced.
Also, as mentioned above, the use of the described controlled transition bridge arrangement requires the use of DC link capacitances. In practice these DC capacitances have to be very large to prevent unacceptable voltage ripple.
Further, the voltages of the cells of the chain-link circuit converter need to be controlled in use to ensure correct operation.